This invention relates to an air/fuel ratio control system for feedback control of the air/fuel ratio of a mixture being supplied to an internal combustion engine, and more particularly to a device provided in such system for effecting air/fuel ratio control during deceleration of the engine.
An air/fuel ratio control system is already known which carries out feedback control of the air/fuel ratio of a mixture being supplied to an internal combustion engine by driving by means of a pulse motor an air/fuel ratio control valve which is arranged to control the air/fuel ratio of the mixture produced by the carburetor, in response to a signal outputted from an O.sub.2 sensor made of zirconium oxide or a like material and provided in the exhaust system of the engine for detecting the concentration of oxygen present in the exhaust gas.
When the throttle valve is suddenly closed to decelerate the engine, there occurs a sudden decrease in the intakeair quantity to cause a corresponding drop in the intake pressure (absolute pressure) in the intake manifold, namely, a corresponding increase in the intake negative pressure. As a consequence, the mixture becomes too rich to cause incomplete combustion within engine cylinders so that the exhaust gas contains unburned HC in large quantities.
If the above-mentioned air/fuel ratio control system continues its feedback control operation during such engine deceleration, the proper concentration of oxygen present in the exhaust gas cannot be detected with accuracy by the O.sub.2 sensor provided in the exhaust system, owing to the increased unburned HC, which makes it impossible to control by means of feedback the air/fuel ratio of the mixture to the theoretical value with accuracy.